The present invention relates to a method for controlling ignition of an internal combustion engine as a function of engine rpm (revolution per minute), especially of a two-stroke engine in hand-held working tools such as motor chain saws etc., as well as to a device for performing the inventive method.
In hand-held working tools the drive motor is in most cases a two stroke engine that is provided with a magnetic ignition device. A magnet wheel, which rotates with the crankshaft of the internal combustion engine, induces in a corresponding induction coil a sine-shaped alternating current, the shape and magnitude of which is directly dependent on the RPM. In order to achieve an ignition that is depending on the number of rpms, it is known to employ the amplitude of the induced voltage for triggering the ignition point. For this purpose, the induced voltage is compared to a threshold value in order to trigger the ignition upon reaching the threshold value.
In order to ensure over a wide adjustment range ignition as a function of engine RPM, additional adaptation measures must be taken, for example, the threshold value for the ignition must be varied. This entails a high technical expenditure so that such adjustable ignition devices are often used only in conjunction with high quality working tools. For simpler embodiments of the working tools it is conventional to make the ignition dependent exclusively on reaching the threshold value, i.e., to make the ignition dependent on the amplitude of the induced voltage. Since the amplitude is greatly dependent on temperature, the air gap, magnetization differences, and other parasitic influences, the resulting ignition point is thus very unreliable.
It is therefore an object of the present invention to provide a method for controlling ignition of an internal combustion engine as a function of engine rpm which method ensures with simple components and a simple construction a reliable, variable ignition point adjustment. Furthermore, a device for performing the inventive method is to be provided.